(a) Field of Invention
This invention relates to drill pipe used in earth boring wherein fluid is circulated down the drill pipe and up the annulus between the pipe and well bore. More particularly the invention relates to protection of the box tool joint member at the upper end of each length of drill pipe by means of hard facing welded thereon, the resulting product being particularly suitable for air drilling, as in geothermal wells.
(b) Description of the Prior Art
(i) Hard Faced Box Tool Joint Members
The composite Catalogue of Oilfield Equipment & Services, 1976/77 Edition, at pages 3216-19 and pages 4994-5 illustrates box tool joint members with tapered elevator shoulders and various configurations of hard facing, typically tungsten carbide, just above the shoulder and extending downwardly thereover to some extent. See also U.S. Pat. No. 3,067,593--McCool which shows hard facing over the upper part of the elevator shoulder and adjacent large diameter portion of a box tool joint connector.
The foregoing catalogue references are perhaps representative of the commercial phase of the prior art. The hard facing may be set into grooves with the outer surface of the hard facing flush with the adjacent parts of the tool joint member or with the hard facing slightly protuberant. The hard facing does not extend downwardly far enough to completely cover the elevator shoulder and it does not cover any part of the weld neck by which the tool joint member is connected by welding to a tube to make a drill pipe. Although the weld between tube and weld neck is heat treated to overcome the effects of the heat of welding on the weld metal and the metal adjacent to the weld, the tool joint is not heat treated adjacent to the hard metal after the hard metal is welded on. Since the hard facing is confined to thick portions of the tool joint member, there is no concern that the tool joint will be unduly affected by the heat of welding during application of the hard facing.
(ii) Heat Affected Zone
That is not to say however that welding does not adversely affect the steel which is welded, especially in the case of welding on hard facing. Hard facing, e.g. tungsten carbide, is usually applied by the MIG (metal-inert gas) process of electric welding. The electrode is made of the desired binding material, such as mild steel. The hard facing is in the form of a powder and is funneled to the weld area. The electrode and metal to be hard faced are heated by passing an electric arc between them to bring the surface of the metal to be hard faced to fusion temperature and to melt the metal of the electrode. The hard metal is deposited on and adhered to the metal to be hard faced. Since the arc temperature is of the order of several thousand degrees F., the metal to be hard faced may be heated well above the critical temperature. Since the tool joint member is made of alloy steel, subsequent cooling of the weld in the air effects an air quench and hardens the steel to the point of making it brittle. The temper effected in the steel of the joint member following its previous heat treatment is thus lost. Also, since the steel is heated well above the critical temperature, the metal in the heat affected zone has a large grain size. Finally, the uneven heating of the metal is apt to cause small cracks in the metal in the heat affected zone. It is only because the part of the tool joint member above the tapered shoulder is so much thicker than the drill pipe tube that the bad effects of welding on hard facing can be tolerated. This is in contrast to the weld between the weld neck and drill pipe tube which, even though the tube end is upset to give great thickness, is preferably reheated above the critical, quenched, and tempered. In this regard, see U.S. patent application Ser. No. 814,542 filed July 11, 1977, by Jimmie Brooks Bolton, now U.S. Pat. No. 4,181,845 assigned to the same assignee as the present application, and the prior art cited therein.
(iii) Geothermal Drilling & Box Tool Joint Members with Ablative Layer
In geothermal well drilling, air must be used at least as the final drilling fluid in order to avoid sealing off the steam bearing formation being drilled. To effect removal of detritus with air, especially at the several thousand foot depths of geothermal wells, rather high air velocity must be employed. High velocity, detritus bearing, air is very abrasive and causes the tool joint shoulder in the drill string to be rapidly eroded, especially in sharp formations. If steam is found, it mixes with the drilling fluid and makes it even more destructive and corrosive. The usual tungsten carbide hard facing may do more harm than good; the roughness is believed to cause turbulence and consequently even more rapid erosion of the steel between the hard particles, resulting in even more roughness, erosion, and loss of the hard particles.
Efforts have been made to counter such wear by covering the entire tapered shoulder with an ablative layer of mild steel. As the ablative layer wears off, more mild steel is welded on, and this is done at frequent intervals. Both the initial layer and the replacement layers are applied in the field rather than at a factory. Plain box tool joint members having no hard facing on the elevator shoulder, only on the large diameter body portion of the box member, are used, because the usual tungsten carbide hard facing on the shoulder would initially be covered up by the ablative layer and would be of no value when the ablative layer wears off, as explained above.
(iv) Difficulties With Ablative Layer
Difficulty is experienced with a box tool joint member carrying an ablative layer. There is a heat affected zone in the base metal below the ablative layer as a result of the heat of the welding operation employed to apply the ablative layer. Since the ablative layer extends over the entire tapered shoulder into the fillet or round joining the shoulder with the weld neck, the heat affected zone extends into the weld neck. The weld neck is the thinnest part of the tool joint member and is subject to failure when the character of its metal is changed, as in the heat affected zone. Such failure may be caused by lowering of the fatigue resistance of the metal in the heat affected zone due to brittleness and increased grain size as previously explained.
Likelihood of failure is also increased due to stress concentration occasioned by difference in materials, the heat affected zone being harder and of different grain structure than the adjacent metal unaffected by heat.
A further source of difficulty with such ablative layer tool joint members lies in the fact that the ablative layer is protuberant. There results a shoulder at the lower end of the ablative layer. There is a stress concentration at the juncture of the shoulder and the weld neck. The weld neck is apt to fail at such juncture.
The shoulder at the lower end of the ablative layer also presents an obstruction to air flow. The resultant eddies cause a ring of erosion in the tool joint neck, just below the ablative layer, and undercutting of the tool joint underneath the lower end of the ablative layer. The tool joint is apt to fail at the eroded areas.
When the tool joint is reconstructed by welding on more ablative material, the ablative material (mild steel) gets into the eroded and undercut parts of the tool joint member causing stress concentration due to the difference in materials of the ablative steel, the base metal, and the heat affected base metal. Also, the heat affected zone adjacent the eroded portions of the tool joint member penetrates farther into the tool joint member, further weakening the member. In addition, the welder may accidentally nick the weld neck below the ablative layer, thereby creating a stress riser in the weld neck. The tool joint member may fail at any of these points of stress concentration or weakness. Finally, the tool joint member may crack during the welding operation or under stress in subsequent use; in the thinner section of the tool joint member at the weld neck the cracks may cause a complete failure, i.e. separation of the tool joint member.